Use SRWLOCK_INIT instead of InitializeSRWLock().

Fixes #323.

Change-Id: Id0e0db1506f0f5535ba1b72c3fa714c2c34428ce
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59210
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/util/mutex.h b/util/mutex.h
index e1587d5..158046b 100644
--- a/util/mutex.h
+++ b/util/mutex.h
@@ -66,7 +66,7 @@
 
 #if defined(MUTEX_IS_WIN32_SRWLOCK)
 
-Mutex::Mutex()             { InitializeSRWLock(&mutex_); }
+Mutex::Mutex()             : mutex_(SRWLOCK_INIT) { }
 Mutex::~Mutex()            { }
 void Mutex::Lock()         { AcquireSRWLockExclusive(&mutex_); }
 void Mutex::Unlock()       { ReleaseSRWLockExclusive(&mutex_); }